Image forming devices that utilize the electrographic method have been widely used, for example in home offices, or by general users. In order to respond to such utilization in the home offices or by the general users, cost reduction, a longer life-span, downsizing, and stability during operation may be required. In order to realize a longer life-span of an image forming device, wearing of functional materials associated with their use may be minimized. For example, for a photosensitive body, which is an image supporting body, surface wear caused by being contacted by corresponding members in a charging process, a developing process, a transferring process, and a cleaning process, respectively, may be considered. It has been known to provide a suppressive measure for suppressing the wear, such as an application member for applying a lubricant agent, so as to prevent a surface of a photosensitive body from being worn. However, as the photosensitive body is downsized in accordance with the downsizing of the device, it is becoming difficult to arrange such a suppressive measure for suppressing the wear. Therefore, recently, various methods are considered such that an outer additive agent including a lubricant component is added to toner and a friction coefficient of the surface of the photosensitive body is reduced.
On the other hand, in order to stabilize the long term operation of the developing device, an amount of the toner corresponding to the amount of the toner consumed during the image development may be supplied. Since the amount of the toner stored in the developing unit is reduced as the developing unit is used, a residual quantity detection unit may be utilized to detect whether the remaining amount of the developer is greater than or equal to a predetermined amount. It has been known to supply the toner based on a detection result by such a residual quantity detection unit. For example, Patent Document 1 (Japanese Published Unexamined Application No. 2011-002526) discloses a two-axis developer circulation type image developing device such that two developer conveyance members are arranged at an upper portion and at a lower portion of a developing unit storing a one-component developing agent. A developer reservoir is continuously provided at an upper portion of an extending portion of the upper developer conveyance member, which is an upstream end portion in the conveyance direction. A residual amount detection unit is arranged at the developer reservoir. The residual amount detection unit optically detects a surface of the developer through a translucent detection window arranged at a side wall of the developer reservoir. With this, the residual amount detection unit determines the residual quantity of the developer.
However, when an outer additive agent including a lubricant component is added to toner, an adhesive force between particles of the toner increases and a cohesive property of the toner is increased. Thus the fluidity of the toner is reduced. When the fluidity of the toner is reduced, the surface of the toner tends to be uneven and the surface of the toner tends not to be formed stably at a suitable position corresponding to the amount of the developer. Therefore, the detected amount of the toner detected by the residual amount detection unit that optically detects the surface of the developer through the detection window tends to be varied. For example, the residual amount detection unit may not detect the surface of the developer when the residual amount of the developer is less than or equal to the predetermined amount, or the residual amount detection unit may detect the surface of the developer when the residual amount of the developer is greater than the predetermined amount. Consequently, the image may be blurred because of the insufficient amount of the toner, or the clogging of the toner may occur because of the overflow of the toner.
It is an objective of the image developing device disclosed in Patent Document 1 to detect a residual amount of the developer within the developing unit by a simple and less expensive configuration. However, in this case, the image developing device tends to be large because the developer reservoir is continuously arranged at the upper portion of the extending unit of the upper developer conveyance member. Thus such a configuration is not suitable for downsizing of the device. Further, in order to detect the residual amount of the developer, an amount of the developer that reaches the developer reservoir may always be required. Therefore, a greater amount of the developer may be required and the cost is increased. Further, when a developer having a low fluidity is used so as to respond to the longer life-span, an excessive amount of the developer within the developing unit may lead to a breakage of the developer conveyance member that is caused by a torque load, or a destruction of the device that is caused by the clogged toner.
The embodiment of the present invention is developed in view of the above problems. An objective of the embodiment is to provide an image developing device, a process cartridge which utilizes the image developing device, and an image forming device which utilizes the image developing device that can properly detect an amount of a developer within the image developing device, that can prevent an image from being blurred due to an erroneous detection, that can prevent clogging of toner, and that can maintain high image quality for a long time, when a developer having a low fluidity is utilized so as to respond to a longer life-span.